The present invention relates to a current mirror circuit used in a semiconductor integrated circuit.
As a conventional highly precise current mirror circuit, the Wilson-type current mirror circuit as shown in FIG. 4 or a circuit disclosed in Japanese Patent Application Laid-Open No. 5-37260 is available.
However, in these conventional current mirror circuits, a highly precise output current is obtained only when a current ratio between an input current and an output current is 1:1. In a case where an input current and an output current are different, or a current mirror circuit is designed to obtain a plurality of output currents from one reference current, the following problems arise.
FIG. 4 is a circuit diagram showing the Wilson-type current mirror circuit having a different current ratio between an input current and an output current. Referring to the circuit shown in FIG. 4, the emitter area ratio of transistors 2 and 3 serving as a reference is set to 1:N to attain a current ratio 1:N, and the emitter area ratio in transistors 4 and 5 is set to 1:N in correspondence with each current. Reference numeral 1 denotes a power supply line, and 12 denotes a reference current source. In the circuit shown in FIG. 4, a collector voltage of the transistors 2 and 3 serving as a reference is evenly controlled by the transistors 4 and 5. Therefore, current variation caused by early voltage of the transistors 2 and 3 can be suppressed. However, the advantage of Wilson-type current mirror circuit, that is, reduction of errors in the base current of each transistor, cannot be attained.
Herein, assume that the base current of the transistors 2 and 4 is Ib and the base current of the transistors 3 and 5 is Nxc3x97Ib, the relation of an input current and an output current is expressed by the following equation (1).
IOUT=Nxc2x7IINxe2x88x92(N2xe2x88x921)Ibxe2x80x83xe2x80x83(1)
As can be seen from equation (1), (N2xe2x88x921) times Ib which is a base current (Ib) of a transistor on the side of the reference current source, as expressed as the second term of the right side of the equation, acts as an error against a desired output current (NIIN). Herein, assuming that a collector current of the transistor 2 is Ic, and a current amplification factor is xcex2, equation (1) is expressed by the following equation (2).
xe2x80x83Iout=Nxc2x7IINxe2x88x92(N2xe2x88x921)Ic/xcex2xe2x80x83xe2x80x83(2)
As can be seen from equation (2), the current amplification factor (xcex2) of the transistor, which appears in the second term of the right side of the equation is a variation factor in the manufacturing process. Therefore, the current amplification factor varies if the quality of transistors varies, and as a result, the output current is largely influenced. Because of this, the conventional Wilson-type current mirror circuit is unable to structure a highly precise current mirror circuit.
Furthermore, FIG. 5 is an example of a current mirror circuit structured such that a plurality of output currents are obtained. In FIG. 5, reference numerals 15 and 16 denote transistors. In this case also, an effect of base current compensation cannot be achieved, similar to the circuit shown in FIG. 4 as an example.
The present invention is made in consideration of the above situation, and has as its object to provide a current mirror circuit which suppresses the influence of early effect of a transistor serving as a reference and which has an effect of base current compensation in a current mirror circuit having a different current ratio between input current and output current or in a current mirror circuit which obtains a plurality of output currents, and an inkjet printing apparatus using the current mirror circuit.
According to one aspect of the present invention, the foregoing object is attained by providing a current mirror circuit utilizing a common base transistor for an output stage, wherein a current obtained by subtracting a base current of the common base transistor of the output stage from a sum of a base current of a first transistor inputting a reference current and a base current of a second transistor where the base of the first transistor and the base of the second transistor are commonly connected, is added to a current output terminal at the same current ratio as an input/output current ratio of the current mirror circuit.
According to another aspect of the present invention, the foregoing object is attained by providing a current mirror circuit comprising a first transistor of a first conductive type where a collector is connected to a reference current source and a base is commonly connected, wherein an emitter and a base of a second transistor of the first conductive type are respectively connected to between the base and the collector of the first transistor, a terminal on the reference current side of the first current mirror circuit is connected to a collector of the second transistor, a collector of a third transistor of the first conductive type, which is commonly connected to the base of the first transistor, is connected to an emitter of a fourth transistor of the first conductive type, a base of a fifth transistor of a second conductive type is connected to the collector of the first transistor, an emitter of the fifth transistor is connected to a base of the fourth transistor and a terminal on an output side of the first current mirror circuit, a collector of the fifth transistor is connected to a terminal of a reference current side of a second current mirror circuit, a terminal on an output side of the second current mirror circuit is connected to a collector of the fourth transistor, and an output current is obtained from the connection point.
According to still another aspect of the present invention, the foregoing object is attained by providing an inkjet printing apparatus comprising a current supply circuit which includes a current mirror circuit having the above-mentioned construction.
In a case where the inkjet printing apparatus have a printhead integrating a rank resister for detecting a resistance indicating a characteristic of the printhead, the current supply circuit preferably supplies an electric current into the rank resistor.
Also, in a case where the inkjet printing apparatus have a printhead integrating a temperature sensor for sensing a temperature of the printhead, the current supply circuit preferably supplies an electric current into the temperature sensor.
The invention is particularly advantageous since an error caused by the early effect and an error in a base current are reduced and a highly precise current mirror output is obtained in a current mirror circuit having a different mirror ratio or in a current mirror circuit having a plurality of output currents.